Josen is the phase in a series of matches between two players - like a jubango, where one player takes Black all the time. This is equivalent to a difference of one stone, or a "one-stone-handicap game".
If three games in a row are won by the same player, or some other threshold is reached, the handicap would shift to two stones. In confrontations between top players, players were spared the embarrassment, for the series would have been suspended long before even the josen phase. A game which if lost would result in a shift in the handicap is called a kadoban.
Between amateur players handicaps usually change one stone at a time. Between professionals the change was much smaller, see JubangoHandicaps for more information.